The present invention relates to plasma display panels (PDPS) and more particularly to a method and apparatus for reducing dynamic false contour in PDP by decrease visual concentration difference.
Conventionally, an image shown on PDP is generated by a control circuit which is enabled to control the number of sustain pulses of red (R), green (G), blue (B) discharge cells of each constituent pixel of PDP based on image data. Hence, gray scale of image may be shown in pixel. This means that color of each pixel is a mixture of brightness and associated color continuously generated by cells. Hereinbelow throughout the specification an image shown on PDP is defined as a field. In general, a continuous sustain pulse of a field on typical PDP is distributed to several sub-fields as shown in FIG. 1. The number of sustain pulses of one sub-field is different from that of the other. In showing a field on PDP, value of gray scale represented by each discharge cell is a combination of gray scales of all constituent sub-fields based on data of image to be shown. Thereafter, a complete field is formed by the sub-fields, thereby showing a desired gray scale. This is the principle of PDP displaying.
On PDP, in showing a field, value of gray scale represented by each cell is depending on data of image to be displayed. Based on rules shown in FIG. 1, it is possible of defining the number of sustain pulses of one sub-field corresponding to discharge cell. Hence, within a unit time required for showing a field in PDP, discharge cell of each sub-field may discharge based on the following typical parameters and weight of the number of sustain pulses thereof:
SF0:SF1:SF2:SF3:SF4:SF5:SF6:SF7=1:2:4:8:16:32:64:128 
However, frequently there is a contour phenomenon caused by interlaced gray scales on portions of image while dynamically showing image on the typical PDP. Such phenomenon is called dynamic false contour. As understood that dynamic false contour may greatly reduce quality of image shown on PDP. Referring to FIG. 2, two continuous dynamic images are exemplified to discuss dynamic false contour wherein two adjacent cells have gray scales of 127 and 128 respectively. In detail, PDP utilize a time division technique to control number of sustain pulses of each cell for showing various gray scales (FIG. 1). Also, eyes of viewer may move as image moves. Hence, a trace of the dynamic image is generated on each point of retina. As a result, each point on retina may track image having different gray scales (FIG. 2). Referring to FIG. 3, hence when viewer watches two continuous dynamic scenes having gray scales of 127 and 128 on two adjacent cells respectively, gray scale of 127 will be sensed by R0 and R1 points of retina with respect to one cell, gray scale of 128 will be sensed by R3 and R4 points of retina with respect to the other cell, and gray scale of 0 will be sensed by R2 point of retina with respect to both cells (i.e., no gray scale) respectively. It is seen that there is a significant drop of sensed gray scale from R1 to R2 and from R2 to R3 with respect to scene represented by two adjacent cells respectively. For image sensed by eyes, interlaced gray scales (i.e., intermittent contour) occur on border between two adjacent cells having gray scales 127 and 128 respectively. This is so-called dynamic false contour.
For further explaining dynamic false contour a coefficient of visual concentration is defined below by PDP designers and manufacturers:
coefficient of visual concentration=(t1m1+t2m2+t3m3+. . . )/(m1+m2+m3), 
where m1, m2, m3, . . . are weights of sub-fields and t1, t2, t3, . . . are time from beginning to midpoint during sustain period in each sub-field. This is best illustrated in FIG. 4. In view of above calculated coefficient, it is found that when visual concentrations of gray scales of two adjacent cells are proximate dynamic false contour does not tend to occur. Hence, by analyzing coefficient of visual concentration between two adjacent cells on PDP those skilled in the art may employ a suitable technique to solve the dynamic false contour based on variation therebetween. In the disclosure of Japanese Patent Laid-open Publication No. 8-270,869, two sets of different coefficients of visual concentration are utilized to exhibit gray scale of each gray scale on PDP by a following technique wherein parameters and corresponding number of continuous sustain pulses are defined with respect to each cell:
SF0:SF1:SF2:SF3:SF4:SF5:SF6:SF7=1:2:4:8:16:24:32:40 
Hence, on PDP as for two sets of coefficient of visual concentration gray scale of 39 is exhibited, i.e.:
1+2+4+8+24=39; 
and
1+2+4+32=39 
Similarly, as for three sets of coefficient of visual concentration gray scale of 40 is exhibited, i.e.:
8+32=40; 
16+24=40; 
and
40=40 
In view of above patent, gray scale exhibited on PDP may be one of multiple sets of coefficient of visual concentration having different combinations as shown in FIG. 5. For solving dynamic false contour it is possible of dividing gray scales having different combinations into two sets of gray scale having different coefficients of visual concentration (e.g., A and B series) based on visual concentration. Further, an average value is obtained from visual concentrations of the sets of gray scale. The average value is taken as a parameter for solving dynamic false contour. As a result, visual concentration difference of gray scale between two adjacent cells is reduced. Referring to FIG. 6, adjacent pixels can exhibit gray scales having sets of different coefficients of visual concentration on PDP as disclosed by the above patent. As a result, visual concentration is more average for substantially eliminating dynamic false contour. In brief, such technique may smooth visual concentration and generate less obvious dynamic false contour. However, as understood that various gray scales exhibited by cells of PDP are determined by the number of discharge. Hence, it is disadvantageous for the discharge of PDP by utilizing two sets of gray scale having different coefficients of visual concentration to exhibit gray scale on each cell.
Thus, it is desirable to provide a method and apparatus for reducing dynamic false contour in PDP by decreasing visual concentration difference in order to overcome the above drawbacks of prior art.
It is therefore an object of the present invention to provide a method for reducing dynamic false contour in a plasma display panel (PDP) comprising the steps of selecting gray scales of different visual concentration series from all of gray scales available to be shown on said PDP to form a visual concentration conversion table, selecting at least one of said visual concentration series as a virtual visual concentration series, converting the original input value of gray scale of each discharge unit into corresponding gray scales of different visual concentration series and virtual visual concentration series while showing each field of a dynamic image on said PDP, showing said converted gray scales on corresponding discharge units corresponding to each sub-field of each field, wherein said gray scales are selected to show the same value of gray scale based on the number of sustain pulses corresponding to values of gray scales of different visual concentration series and virtual visual concentration series.
In one aspect of the present invention, visual concentration of different value of gray scale shown by any two adjacent discharge units on the dynamic field is averaged to obtain a value of gray scale having a smaller visual concentration difference. This can substantially eliminate dynamic false contour on PDP due to larger visual concentration difference.
The above and other objects, features and advantages of the present invention will become apparent from the following detailed description taken with the accompanying drawings.